It is about how many contributors out of total number are doing this microstock seriously.
It is based on ranking feature information on Fotolia.
I started uploading there a year ago with some 2000 pictures. After my first few batches of 50 pictures and few sales my Overall Ranking was 35 000. So, I guess there were around 40 000 contributors. In few months period I uploaded all of my pictures and started making regular sales, but still nothing great. I jumped really fast to 3000 position. And during this year I made it very slowly to position 1000.
I was surprised to see that there are only around 3000 people making good income out of this and 35 000 or so playing minor role.